design property: replace try/catch by Enum.IsDefined

Delphin0850 Merge for Headers and Footers margin attribute

issue #63 - fixed FontSize() method in Paragraph.cs to accept half sizes

Added the independent Font implementation

Added unit test Test_Table_SetTableDesignNone() draft. Requires fix in Tables.

Code cleanup - introducing the 'nameof' operator instead of string values

Fixed insertColumns/removeColumns method along with remove. It is now able to work with merged cells.

Added a method to delete cells

Added gridSpan property to cells

Added gridAfter property to rows

Able to add columns to tables where each row have different amount of
cells

Allowing InsertColumn method take into account merged cells for multiple
row add columns where the count of cells is less than the index but is
not less table column count

Fix for failing FontFamily name test

On mono the actual name of a FontFamily may be different from the alias used to create the FontFamily.

Fix for #17

On mono, the OriginalString of a Uri sometimes is prefixed with file:// leading to issues when combining paths. We strip the prefix if necesary to avoid that.

Adding configurations for AppVeyor and Travis CI

Fixed Test_Document_Paragraphs

Also moved the documents directory to a proper temprorary path.

Changed Relative Paths in Unit Tests

Migration from MSTest to NUnit

Using AppDomain.CurrentDomain.BaseDirectory instead of Assembly.GetExecutingAssembly().Location to the determine the location of .docx files and other resources. This method is invariant under the test runner in use.
Resources are set to "Copy if newer" to include them during build.

Fix HelperFunctions.FormatInput doubles new lines (Issue #44)

fixed lastChar initialization

fixed lastChar assignment