ENH: Add variance forecasting

[bashtage]

Add variance forecasting to the forecast function[

• Constant Variance Forecasting
• GARCH Forecasting
• EGARCH Forecasting
• HARCH Forecasting
• EWMA Forecasting
• EWMA2006 Forecasting
• Documentation
• IPython notebook
• Hedgehog plot
• Test first_obs, last_obs and hold_back interaction with forecasting

[codecov-io]

Current coverage is 97.61% (diff: 97.25%)

Merging #132 into master will increase coverage by 0.19%

@@             master       #132   diff @@
==========================================
  Files            35         37      +2   
  Lines          6274       7626   +1352   
  Methods           0          0           
  Messages          0          0           
  Branches          0          0           
==========================================
+ Hits           6112       7444   +1332   
- Misses          162        182     +20   
  Partials          0          0           

Powered by Codecov. Last update 556e145...fb23342

[coveralls]

Coverage increased (+0.008%) to 96.528% when pulling 2da2daa on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.7%) to 97.259% when pulling 56e8c8f on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.7%) to 97.259% when pulling 56e8c8f on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.5%) to 97.016% when pulling 390ad26 on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.5%) to 97.016% when pulling 9ba0fc6 on variance-forecasting into 556e145 on master.

[coveralls]

Coverage decreased (-2.0%) to 94.533% when pulling b1f4f3b on variance-forecasting into 556e145 on master.

[coveralls]

Coverage decreased (-2.0%) to 94.533% when pulling b1f4f3b on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.08%) to 96.601% when pulling 609839f on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.08%) to 96.601% when pulling 609839f on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.08%) to 96.601% when pulling c0b9928 on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.08%) to 96.601% when pulling fb23342 on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.08%) to 96.601% when pulling fb23342 on variance-forecasting into 556e145 on master.

[coveralls]

Coverage increased (+0.5%) to 97.001% when pulling 637f06a on variance-forecasting into 556e145 on master.