style updates respecting Bootstrap defaults

Updated CSS to match Bootstrap defaults:
* closes #195: remove margin-bottom from .bootstrap-tagsinput
* closes #199: match input.form-control padding and placeholder color

before:
![image](https://cloud.githubusercontent.com/assets/906846/3884965/8ec9649c-21b8-11e4-9963-612a197e6814.png)

after:
![image](https://cloud.githubusercontent.com/assets/906846/3884967/a1e85f1a-21b8-11e4-8150-3a07ca4ba8f6.png)