Updating LMS Login button to line up SASS with Drupal Login button

This PR is in response to the acceptance criteria feedback on the following JIRA Ticket:
https://openedx.atlassian.net/browse/ECOM-64

Quick PR to change the LMS Login button such that:
* It has black text with a gray border when you are on the login page
* It has black text with a blue border when you are hovering over the login button
* It has black text with a gray border when you are on the login page, and hovering over the login button

Normal State: 
![image](https://cloud.githubusercontent.com/assets/235752/4049667/b1344d10-2d4f-11e4-851e-e282871ba307.png)

Selected State: 
![image](https://cloud.githubusercontent.com/assets/235752/4049672/bbcbfd2c-2d4f-11e4-8705-579e23bea207.png)

Hover State: 
![image](https://cloud.githubusercontent.com/assets/235752/4049675/c3b86188-2d4f-11e4-90c2-5a54e0339609.png)

@talbs  FYI on SASS
@dianakhuang @wedaly @flowerhack on LMS review
@griffresch Confirm expected styling and state. 
You can review on the following sandbox:
http://steve.m.sandbox.edx.org/login
http://steve.m.sandbox.edx.org/register