Add consistency checks to expensive operations

[chiraganand]

Since the TS object can be manipulated externally we need to perform consistency checks to make sure certain conditions are not violated before any expensive operation (such as apply(), rollapply(), or even print()).

Conditions which should be checked:

1. The index should always be sorted.
2. Duplicate values in the index should be checked if the object doesn't support them. Whether the object supports duplicate values or not can be stored inside some sort of a metadata object without touching the TS struct. This is until DataFrames.jl starts supporting metadata.