Add PlatformToolset ver. to msvs proj template

This patch adds support for an easier syntax for customizing which
version of Visual Studio msvs.py targets. It also allows users to
specify the PlatformToolset property on projects generated by waf. This
is necessary in order to properly generate projects for newer versions
of Visual Studio such as 2013 and 2015.

Example Syntax:

To generate a solution targeted at visual studio 2013 use the following
example:

```Python
class msvs_2013(msvs.msvs_generator):
    cmd = 'msvs2013'
    numver = '13.00'
    vsver = '2013'
    platform_toolset_ver = 'v120'
```

This will populate the associated fields correctly for msvs 2013.

waflib/extras/msvs.py

@@ -105,7 +105,7 @@ PROJECT_TEMPLATE = r'''<?xml version="1.0" encoding="UTF-8"?>
 	<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='${b.configuration}|${b.platform}'" Label="Configuration">
 		<ConfigurationType>Makefile</ConfigurationType>
 		<OutDir>${b.outdir}</OutDir>
-		<PlatformToolset>v110</PlatformToolset>
+		<PlatformToolset>${project.platform_toolset_ver}</PlatformToolset>
 	</PropertyGroup>
 	${endfor}
 
@@ -509,6 +509,7 @@ class vsnode_project(vsnode):
 		self.path = node
 		self.uuid = make_uuid(node.win32path())
 		self.name = node.name
+		self.platform_toolset_ver = getattr(ctx, 'platform_toolset_ver', None)
 		self.title = self.path.win32path()
 		self.source = [] # list of node objects
 		self.build_properties = [] # list of properties (nmake commands, output dir, etc)
@@ -715,6 +716,9 @@ class msvs_generator(BuildContext):
 	'''generates a visual studio 2010 solution'''
 	cmd = 'msvs'
 	fun = 'build'
+	numver = '11.00' # Visual Studio Version Number
+	vsver  = '2010'  # Visual Studio Version Year
+	platform_toolset = 'v110' # Platform Toolset Version Number
 
 	def init(self):
 		"""
@@ -744,8 +748,9 @@ class msvs_generator(BuildContext):
 		if not getattr(self, 'vsnode_project_view', None):
 			self.vsnode_project_view = vsnode_project_view
 
-		self.numver = '11.00'
-		self.vsver  = '2010'
+		self.numver = self.__class__.numver
+		self.vsver  = self.__class__.vsver
+		self.platform_toolset_ver = self.__class__.platform_toolset_ver
 
 	def execute(self):
 		"""
@@ -980,6 +985,8 @@ class msvs_2008_generator(msvs_generator):
 	'''generates a visual studio 2008 solution'''
 	cmd = 'msvs2008'
 	fun = msvs_generator.fun
+	numver = '10.00'
+	vsver = '2008'
 
 	def init(self):
 		if not getattr(self, 'project_extension', None):
@@ -997,8 +1004,6 @@ class msvs_2008_generator(msvs_generator):
 			self.vsnode_project_view = wrap_2008(vsnode_project_view)
 
 		msvs_generator.init(self)
-		self.numver = '10.00'
-		self.vsver  = '2008'
 
 def options(ctx):
 	"""