Add LGTM code quality badges

[xcorail]

Hi there!

I thought you might be interested in adding these code quality badges to your project. They will indicate a very high code quality to potential users and contributors.
To get an idea of the analyses reflected by these grades, check the alerts discovered by LGTM.

N.B.: I am on the team behind LGTM.com, I'd appreciate your feedback on this initiative, whether you're interested or not, if you find time to drop me a line. Thanks.

[coveralls]

Coverage remained the same at 95.163% when pulling 6fdd9f6 on xcorail:master into 2a5b114 on QuantEcon:master.

[mmcky]

hey @xcorail thanks for the PR. Your project looks pretty neat -- I will take a closer look this week.

[xcorail]

Hey @mmcky

Hope you'll find it useful!
Some precisions on how / when the analysis is triggered:

• The recommended usage is to activate PR integration. LGTM will then analyse the PR immediately and report any new problem (or fix) directly within the PR. It's a one-click operation
• Otherwise LGTM polls your repo once a day and analyses all commits, most recent first

Cheers

[mmcky]

thanks @xcorail -- I will check that out the integration.