Improve start/end utilization in TradingCalendar

[Peque]

Changes:

• Allow to pass string and datetime types to TradingCalendar's start and end parameters.
• Set default parameters to None. This way it is easier to propagate undefined start/end parameters when the TradingCalendar initialization is encapsulated in another function/method (no need to import start_default/end_default to have consistent behavior).
• Store start/end attributes as they are useful to know the first/last day of the calendar (they do not need to match the first/last trading session in the calendar).

[coveralls]

Coverage increased (+0.005%) to 87.552% when pulling 50486e5 on Peque:flexible_startend into c2e0114 on quantopian:master.

[coveralls]

Coverage increased (+0.007%) to 87.554% when pulling 6690b31 on Peque:flexible_startend into c2e0114 on quantopian:master.

[coveralls]

Coverage increased (+0.007%) to 87.554% when pulling 6690b31 on Peque:flexible_startend into c2e0114 on quantopian:master.

[coveralls]

Coverage increased (+0.007%) to 87.554% when pulling 9933da3 on Peque:flexible_startend into beba243 on quantopian:master.

[coveralls]

Coverage increased (+0.007%) to 87.554% when pulling 9933da3 on Peque:flexible_startend into beba243 on quantopian:master.

[freddiev4]

x-ref #418

[ernestoeperez88]

TradingCalendar has been moved to its own repo: https://github.com/quantopian/trading_calendars