Add flow typedefs

[amiralies]

Closes #1395

[xcarpentier]

Hi,
Sorry, but can you resolve conflicts?
I will merge it after your resolving.

[amiralies]

there is no conflict with master branch

[xcarpentier]

Hi @amiralies,
Thanks for this PR!
I try to find a way to automate it because I didn't use flow and I won't update it myself, I think.
To keep it up to date, what do you think if we use this solution: https://github.com/joarwilk/flowgen?
Do you have some time to investigate it?

[amiralies]

@xcarpentier flowgen doesn't convert all types properly, sepcially external libs such as React native and React.
We can add some readme about updating flow types and say that flow types are not guaranteed to always be updated.

[amiralies]

@xcarpentier can you release alpha / rc version as next realse on npm so i can test and expermient with flow types?

[xcarpentier]

You can test on this version: 0.10.2-0