Provide config option to support uploads to CloudFront
Do not to call getPath() again in makeStringToSign
[README.md] add info about cloudfront config param
Merge pull request #44 from dconnolly/master

Provide config option to support uploads to CloudFront
Add ability to send x-amz-headers with upload and complete

This is needed when using x-amz-security-token, which is a needed header
when using temporary security credentials.
See AWS doc for details:
http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html#UsingTemporarySecurityCredentials
Merge pull request #46 from wes-r/master

Add ability to send x-amz-headers with upload and complete
Update README.md

Added details around two new optional params: **xAmzHeadersAtUpload**,  **xAmzHeadersAtComplete**.
Merge pull request #48 from wes-r/master

Update README.md
Adding /<bucket>/ to the url to sign when in cloudfront

Cloudfront directs queries directed at /resource to S3 in the form /bucket/resource, S3 subsequently thinks the URL to compare signing against has form /bucket/resource rather than /resource, so when providing the URL for signing you need to prepend the bucket name.
Merge pull request #49 from Marogian/master

Adding /<bucket>/ to the url to sign when in cloudfront
add signing headers