Fixes for issues 175 and 179. Version numbers changed to 2.1.0

This pull request fixes several issues:

1. Created wrapper classes for the InstrumentMain, CheckCoverageMain, MergeMain, and ReportingMain classes to preserve backward compatibility (Issue #175) 

2. Moved code from the InstrumentMain, CheckCoverageMain, MergeMain, and ReportingMain classes' main methods to helper methods that don't use System.exit to facilitate integration from build tools (Issue #175)

3. Changed the base directory field of Arguments and ArgumentBuilder from a File that defaults to "." to a String that defaults to null to fix an instrumentation problem (Issue #179)

4. Change the version number of all artifacts to 2.1.0-SNAPSHOT