Import all S3 methods from reshape (including default)
Set the generic for S4 dispatch to use the default S3 method.

This addresses #47. Still needs test cases.
Should use reshape::melt (the original S3 generic) rather  than reshape::melt.default.
Use reshape::melt rather than reshape::melt.default in S4 generic. Fixes #47
Merge branch 'meltfix' of github.com:RGLab/SingleCellAssay into meltfix
test coverage for melt method.  #47