Use better name error; Tweak name requirements

- "Invalid name" was replaced with a description of a valid name.
  Fixes #100
- Tweaked the valid name regex to require at least one number or letter.

chatroom.go

@@ -60,7 +60,7 @@ func (cr *ChatRoom) Join(conn *chatConnection, data common.JoinData) (*Client, e
 	}
 
 	if !common.IsValidName(data.Name) {
-		sendHiddenMessage(common.CdNotify, "Invalid name")
+		sendHiddenMessage(common.CdNotify, common.InvalidNameError)
 		return nil, UserFormatError{Name: data.Name}
 	}
 

common/utils.go

@@ -6,7 +6,9 @@ import (
 	"regexp"
 )
 
-var usernameRegex *regexp.Regexp = regexp.MustCompile(`^[0-9a-zA-Z_-]+$`)
+var usernameRegex *regexp.Regexp = regexp.MustCompile(`^[0-9a-zA-Z_-]*[a-zA-Z0-9]+[0-9a-zA-Z_-]*$`)
+
+const InvalidNameError string = `Invalid name.<br />Name must be between 3 and 36 characters in length; contain only numbers, letters, underscores or dashes; and contain at least one number or letter.<br />Names cannot contain spaces.`
 
 // IsValidName checks that name is within the correct ranges, follows the regex defined
 // and is not a valid color name