StellarTomlResolver

[bartekn]

This PR introduces StellarTomlResolver that is responsible for downloading and parsing stellar.toml files. The API is simple:

StellarSdk.StellarTomlResolver.resolve('acme.com')
  .then(stellarToml => {
    console.log(stellarToml.FEDERATION_SERVER);
    console.log(stellarToml.CURRENCIES);
  });

The second parameter in resolve method: opts allows setting allowHttp option, that will download stellar.toml file over http protocol (instead of https).

[coveralls]

Coverage increased (+0.3%) to 90.435% when pulling a077fbf on stellar-toml-resolver into a105b69 on master.

[nullstyle]

It's worth noting that IMO having the allowHttp option be a per-call option will probably be annoying to use. I can't think of any situation where a developer would want to switch between modes within the same process... more likely, a given process will want the same behavior for all resolve operations: either all over SSL or none over SSL.

IMO, treating this similar to how we treat what network the sdk is configured against is the most appropriate solution.

I would add a module level "allowHttp" variable that will be used in the event that opts doesn't override it.

[coveralls]

Coverage increased (+0.9%) to 89.63% when pulling 2bf5438 on stellar-toml-resolver into c301bb8 on master.

[coveralls]

Coverage increased (+0.9%) to 89.63% when pulling 2bf5438 on stellar-toml-resolver into c301bb8 on master.

[bartekn]

Created Config class to set global config params.