Fixed Disasm.Basic.insn_of_mem function.

Covered the issue with a test suite.
Merge pull request #57 from ivg/fix-insn_of_mem

Fixed Disasm.Basic.insn_of_mem function.
#44 is fixed.
added support for may_store and may_load kinds.
Merge pull request #58 from ivg/add-may_save_load

added support for may_store and may_load kinds.