Added SCON support for special functions

Changes include:

* Strings in SCON format can be included in single or double quotes.
* Dollar sign inside strings must be escaped (\$).
* SCON configuration can contain the following dynamically
  evaluated expressions: ${...}, $date{}, $path{}:

1. Environment variables included in "${...}" or "$env{...}". A special
   "EXEPATH" variable returns path of the currently running executable.
```
config {
    key1 = ${HOME}
    key2 = "${A} $env{B} ${C}"
    key3 = "${EXEPATH}"
}
```

2. Date function takes may contain formatting options acceptable by
strftime(3). The function may additionally take two options:
    - 'now' in the format: "YYYY-mm-dd HH:MM:SS"
    - utc: true | false
```
config {
    key3 = $date{'%Y%m%d-%H:%M:%S', utc=true}
    key4 = $date{'%Y%m%d', now="2014-12-01 23:55:13"}
    key5 = "Time is: $date{'%Y%m%d-%H:%M:%S'}"
    key6 = "Home is: ${HOME}. Time is: $date{'%Y%m%d-%H:%M:%S at ${HOME}'}"
}
```

3. Path function takes a path that may begin with "~" and may
contain date/time formatting options just like the $date{} function:
    - 'now' in the format: "YYYY-mm-dd HH:MM:SS"
    - utc: true | false
```
config {
    key7  = $path{/var/log/%Y%m%d-%H%M%S-mqt.log', utc=true}
    key8  = $path{'${HOME}/var/config-%Y%m%d.log', now="2014-12-01 23:55:13"}
    key9  = "$path{~/tmp/temp%H:%M:%S.log}"
    key10 = "$path{'${EXEPATH}/tmp/temp%H:%M:%S.log'}"
}
```