Define HalfMonthModifiedFollowing as business day convention
Define HalfMonthModifiedFollowing
HalfMonthModifiedFollowing update
exported Half-Month Modified Following to Excel
fixed Half-Month Modified Following bug
Reorder enumeration to preserve old values.

Just in case someone is relying on the numerical values of the
enumeration.
Export new convention to SWIG.
Merge pull request #110.