Fix mispellings/typos in source

This commit corrects four misspellings or typos found in comments in skicka.go
Merge pull request #29 from gusennan/29_Spelling

Fix mispellings/typos in source
Print usage when -h, -help, or --help are invoked

The Usage var in the flag package defines the function that is called
when the flag -help or -h is called but no help flag is defined.  The
flag.Usage variable can be reassigned to another function if an
alternate function should be invoked, which is what this commit does.
If "-h", "--h", "-help", "--help" or "help" is typed as the command line
argument, "usage()" is invoked.
Merge pull request #30 from gusennan/25_HelpText

Print usage when -h, -help, or --help are invoked
Merge branch 'master' of github.com:google/skicka
Fix du: add file's size contribution to all parent directories.

Issue #8.
Modify download function to show progress based on size

This commit changes the algorithm for how the download sync works.
Specifically, it figures out which files must be downloaded before it
starts trying to download anything.  In this manner, it's possible to
calculate how much the total download will be, in contrast to having the
logic whether something should be downloaded or not within the download
function itself.

The upload functionality will come next.
Adds size-sensitive progress bar to uploads

Instead of relying on the file count for determining progress on uploads
and downloads, this commit determines how many bytes will be uploaded
and then sets the progress bar to increase by however many bytes were
uploaded after each file.

To accomplish this, some of the longer functions were split into smaller
components.  For example, it is imperative to be able to know how large
a sync will be before starting the sync, therefore it is not possible to
wait until on the individual file level to have the file check it it
should sync or not.  As a consequence, the control of figuring out what
should be sync'd and then the control of the transport of the bits are
now split into two different areas, whereas before they were one.
Print the error if an error occurs.

Adds the message from the error if and when an error occurs while
checking if a file needs to be downloaded.
Merge pull request #32 from gusennan/15_progress_bar

Run the progressbar based on file transfer size
Merge branch 'master' into chunked-uploads