Fix #2: Add MIT license header

The header has been added to all .cs files except "Microsoft.Exchange.WebServices.Strings.cs", which is an auto-generated file.

The script used to add the license header:
```
#!/bin/bash
for file in $(find -name "*.cs")
do
    echo "$file"
    # Remove old header
    sed -i '1,5d' "$file"
    cat license.txt "$file" > "$file".new && mv "$file".new "$file"
done
```
The inserted header:
```
#region License
 
// Exchange Web Services Managed API
//
// Copyright (c) Microsoft Corporation
// All rights reserved.
//
// MIT License
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
 
// THE SOFTWARE IS PROVIDED *AS IS*, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
 
#endregion
```