Throw error in development when @observable is used on getters

[quanganhtran]

This PR is to address #1454

The variant check is added to the property creator function. The caveat is that it throws when the target is initialized (even if the property being accessed is not the erroneous one).

Fixed random typos in comments that I came across.

• Added unit tests
• Updated changelog
• Updated docs (either in the description of this PR as markdown, or as separate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• Added typescript typings
• Verified that there is no significant performance drop (npm run perf)

[coveralls]

Coverage increased (+0.005%) to 92.975% when pulling 8cca76d on quanganhtran:master into dbe4c93 on mobxjs:master.

[mweststrate]

Thanks!