PLT-492 Indices for Fx

Add FxIndex and standard constants
Represents daily published FX rates
PLT-492 Indices for Fx

Enhance from PR
Merge pull request #35 from OpenGamma/topic/PLT-492

PLT-492 Indices for Fx
Simplify and enhance schedules

Simplify SchedulePeriod, removing frequency, roll convention and type
Add frequency and roll convention to Schedule
Determine schedule type on the fly

Provide additional mechanisms to manipulate a schedule
- whether a schedule is a term period
- get initial/final stubs and regular periods
- merging/splitting a schedule

DayCount ScheduleInfo altered to operate on schedule